A variety of growth factors are synthesized by mammalian cells. These growth factors are synthesized as larger molecules within the cell and are then processed into the smaller, active forms of the growth factors. The active forms are secreted from the cell and interact with other cells. The expression and processing of these growth factors is a complex , multi-step phenomenon. One of these steps is the proteolysis of the precursor. We are investigating the nature of this proteolysis and which enzymes are responsible. Understanding the processing of precursor forms of growth factors and cytokines helps CBER staff evaluate the manufacturing aspects of applications and may help to develop safer and more effective growth factor products.